Application of nitride semiconductors having a high saturation electron velocity and a wide band gap to high-withstand-voltage, high-power semiconductor devices is being considered. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV that is greater than the band gap 1.1 eV of Si and the band gap 1.4 eV of GaAs, and has a high breakdown field strength. For this reason, a nitride semiconductor such as GaN is a very promising material for a high-voltage-operation, high-power semiconductor device for a power supply.
Many reports have been made on field effect transistors, particularly, high electron mobility transistors (HEMT), which are examples of semiconductor devices using nitride semiconductors. For example, as a GaN HEMT, an AlGaN/GaN HEMT, which uses GaN as an electron transit layer and AlGaN as an electron supply layer, is getting attention. In an AlGaN/GaN HEMT, distortion occurs in AlGaN due to a difference between the lattice constants of GaN and AlGaN. The distortion causes piezoelectric polarization and a spontaneous polarization difference of AlGaN, which in turn generate a high-density two-dimensional electron gas (2DEG).
Development of InAlN/GaN HEMTs using InAlN as the electron supply layer has been active in recent years. An InAlN/GaN HEMT has a performance that surpasses the performance of a HEMT using AlGaN as the electron transit layer. In an InAlN/GaN HEMT, the lattice matching between In and GaN can be achieved and a high-quality crystal film can be obtained by setting the composition ratio of In at 17 to 18%. Also, when InAlN is formed with such a composition ratio, the formed InAlN has very high spontaneous polarization. Therefore, an InAlN/GaN HEMT can generate a two-dimensional electron gas (2DEG) having a density that is two to three times greater than the density of the two-dimensional electron gas generated by an AlGaN/GaN HEMT using AlGaN as an electron transit layer. For the above reasons, HEMTs using InAlN as the electron supply layer are getting attention as next-generation high-power devices (see, for example, Japanese Laid-Open Patent Publication No. 2013-89970 and Japanese Laid-Open Patent Publication No. 2013-33877).